(a) Field of the Invention
The present invention relates to a diode-laser side-pumped solid-state laser device and, more particularly, to a solid-state laser device pumped by a semiconductor laser diode to generate a solid-state laser beam with a higher brightness and a higher efficiency.
(b) Description of the Related Art
As a pumping (exciting) scheme for a solid-state laser device such as having a Nd:YAG laser medium, pumping of the solid-state laser device by using a semiconductor laser diode (referred to as simply xe2x80x9claser diodexe2x80x9d hereinafter) is now highlighted due to a higher absorbing efficiency thereof by the laser medium compared to the lamp-pumping scheme for the solid-state laser device. This type of solid-state laser device (SSLD) uses the laser diode as a pumping light source having a longer lifetime, smaller dimensions and higher efficiency.
A variety of SSLDs using a side-pumping scheme have been proposed heretofore, wherein a large number of laser diodes are arranged in an array on the side surface of an elongate solid laser medium, such as a cylindrical solid laser rod, along the lasing axis of the solid laser rod. The laser diode has inherently a linear brightness distribution, which suitably matches with the side-pumping scheme.
FIG. 1 shows a sectional view of a conventional SSLD using the side-pumping scheme, as described in xe2x80x9cIEEE Journal of Quantum Electronicsxe2x80x9d 1992, Vo. 28, No.4, pp977-985. The SSLD has a cylindrical solid laser rod (Nd:YAG laser rod) 1 extending normal to the sheet of FIG. 1. The laser rod 1 is encircled with a cooling tube 3 having an inner diameter larger than the outer diameter of the laser rod 1. The space between the laser rod 1 and the cooling tube 3 is filled with a cooling medium 2 flowing therebetween for cooling the laser rod 1.
In the vicinity of the outer surface of the cooling tube 3, a large number of laser diodes 100a to 100h are arranged, with four laser diodes 100a to 100d being arranged for a unit length of the laser rod 1 and separated from one another by a uniform angular distance with respect to the central axis of the laser rod 1. Other four laser diodes 100e to 100h in another array are deviated from the array of laser diodes 100a to 100h by 45 degrees as viewed along the axis of the laser rod 1. This configuration provides eight pumping directions to improve the axial symmetry of the energy absorption distribution for the pumping laser by the laser rod 1.
In the SSLDs using the side-pumping scheme and described in Patent Publications JP-A-10-326927 and xe2x80x9410-84150, the laser beam emitted by each of the laser diodes diverges to a whole angle as large as 30 degrees in the direction normal to the active layer of the laser diode. Thus, the laser diodes should be disposed in close proximity with the laser rod 1 in order to efficiently emit the laser beam toward the laser crystal.
In the exemplified SSLD of FIG. 1, the distance between the emission end surface of each of the laser is diodes 100a to 100h and the cooling tube 3 is as small as 1 mm. Although the laser diode has a small chip size, the overall dimensions of the laser diode are equivalent to the diameter of the laser rod, because the laser diode has a mount member for the chip and a cooling device such as a Peltier element or cooling water path. This prevents a large number of laser diodes from being disposed for a unit length of the laser rod, and impedes a higher output power of the SSLD.
For alleviating the difficulty of arrangement of a large number of laser diodes in close proximity of the laser rod, it is considered to prevent the divergence of the laser beam from the laser diode by using an optical unit such as a lens, thereby efficiently emitting the laser beams from the laser diodes toward the side surface of the laser rod. In an alternative, it is also known that an elongate optical waveguide encircling the laser rod is provided for guiding the laser beams emitted from a large number of laser diodes toward the laser crystal of the laser rod.
The SSLD of FIG. 1 has a disadvantage in that the power efficiency of the pumping laser beam is relatively low because some of the laser beam passes the laser rod without being absorbed by the laser rod.
FIG. 2 shows another conventional SSLD using the side-pumping scheme, described in xe2x80x9cOptics Letterxe2x80x9d 1995, vol. 20, No. 10, pp1148-1150. In the SSLD, a cylindrical lens (collimate lens) 101a, for example, disposed in close proximity of the laser diode 100a collimates a laser beam component (advanced-phase-axis component), which is normal to the thickness direction of the active layer. This alleviates divergence of the pumping laser beam 106 emitted from the laser diode 100, and allows the pumping laser beam 106 to transmit in the space toward the side surface of the laser rod 1. Thus, a large number (nine at a maximum in this example) of laser diodes 100 can be disposed around the circumference of the laser rod 1 for a unit length thereof due to a large distance between the laser diode 100 and the laser rod 1.
In the SSLD of FIG. 2, a portion of the pumping laser beam 106 (106a or 106b) not absorbed in the laser rod 1 and passing the same is reflected by mirrors 104 (104a or 104b) surrounding the cooling tube 3. For example, the pumping laser beam 106a is irradiated onto the laser rod 1 through a slit formed between adjacent mirrors 104f and 104g, and is absorbed or passed by the laser rod 1. The laser beam passed by the laser rod 1 is then reflected by a corresponding mirror 104a toward the laser rod 1.
The SSLD of FIG. 2 has a disadvantage in that a portion of the laser beam which is not absorbed by the laser rod 1 passes the slit of the mirror member and thus is not recovered for absorption. More specifically, when a parallel ray of the pumping laser beam is incident onto the cylindrical laser rod 1, some of the laser beam not absorbed and passed by the laser rod 1 is focused and then diverged. The diverged laser beam is more likely to pass through the slit without being reflected by the mirror member.
Patent Publications JP-A-11-284256 and xe2x80x9411-284253 describe SSLDs having reflecting mirrors similarly to the SSLD of FIG. 2. FIG. 3 shows the SSLD described in JP-A-11-284256, wherein the advanced-phase-axis component of the pumping laser beam 6 collimated by a rod lens (not shown) is irradiated through a slit 135 formed in a mirror member 134, which is located on the outer periphery of the cylindrical body 133 encircling the solid laser rod 1. The space between the laser rod 1 and the cylindrical member 133 is filled with a cooling medium, and the cylindrical member 133 alleviates the convex lens function of the laser rod 1. A portion of the pumping laser not absorbed by the laser rod 1 is focused at a focal point in the vicinity of the laser rod 1.
The focused laser beam portion 6t is then reflected by a corresponding mirror 134 toward the laser rod 1 after a moderate divergence. The moderate divergence, effected by the alleviation of the convex lens function of the laser rod 1 and shown by a small diameter xe2x80x9cdxe2x80x9d of the laser beam 6, allows the effective reflection area of the mirror member 134 to be maintained larger irrespective of the presence of a number of the slits 135 formed therein.
The SSLD of FIG. 3 has a disadvantage in that the cylindrical body has a larger thickness and thus has a larger weight and a higher equipment cost. In addition, a larger number of laser beams in different directions reduces effective reflection of the mirror member due to the larger number of openings disposed for introducing the laser beams.
Patent Publication JP-A-11-284253 describes the SSLD shown in FIG. 4, wherein a reflecting layer 144 is provided on the outer surface of the cooling tube 3 for reflecting laser beams passed by the laser rod 1 toward the laser rod 1 again. The laser beams are irradiated from the direction where the reflecting layer is not disposed.
The SSLD of FIG. 4 has a disadvantage in that it is difficult to form the reflecting layer having a slit on the outer surface of the cooling tube 3 at a low cost. In addition, a strict alignment accuracy is needed between the pumping laser and the slit, which complicates the fabrication process for the SSLD.
Patent Publication JP-A-4-35077 describes the SSLD shown in FIG. 5, wherein the pumping laser beams 106a to 106c from laser diodes 100a to 100c are transmitted to the laser rod 1 by using respective waveguides 105 instead of optical unit such as lens.
The SSLD has a disadvantage similar to that of the SSLD of FIG. 1.
Patent Publications JP-A-7-94813, 10-135539 and 11-17252 describe SSLDs each using a wedge glass plate (or tapered glass plate) having a larger thickness at the receiving end compared to the emission end, thereby improving the optical coupling efficiency between the laser diode and the glass plate.
FIG. 6 shows the SSLD described in JP-A-8-181368, wherein a pair of laser beams 116a and 116b are irradiated to the side surfaces of the laser rod 1 via a pair of waveguide plates (waveguides) 115a and 115b. Each of the laser diodes (not shown) is coupled to a corresponding waveguide 115a or 115b directly or through a lens. The laser rod 1 and the cooling tube 2 are encircled by a reflecting member 114, which reflects the laser beams a plurality of times until the laser beams are absorbed. This affords the advantage of uniform absorption density of laser beams in the radial direction.
The SSLD of FIG. 6 has a configuration for effectively introducing the pumping laser into the laser rod by preventing the pumping laser from travelling around the laser rod within the space between the laser rod and the cooling tube. For this purpose, the cross-sectional area of the mirror should be equivalent to the cross-sectional area of the laser rod, which is difficult to achieve however, The difficulty arises partly from the presence of the cooling tube separating the laser rod and the mirror surface.
If the equivalence between the cross-sectional areas is not achieved, the amount of the pumping laser not absorbed by the laser rod remains low. In addition, a long time of operation causes deterioration and contamination of the mirror surface, which further increases the amount of pumping laser not absorbed by the laser rod.
FIG. 7 shows the SSLD described in JP-A-10-275952, wherein four laser beams are irradiated toward the laser rod 1 from respective directions. The optical axis of each laser beam is deviated from the central axis of the laser rod 1 by a specified distance 118. The specified distance 118 allows a uniform heat distribution within the laser rode 1. The SSLD of FIG. 7 suffers problems, however, similar to those of the SSLD of FIG. 6.
FIG. 8 shows the SSLD described in JP-A-11-163446, wherein the incident laser beam emitted from the waveguide 125 is deflected due to the inclined surface of the emission end of the waveguide 125 with respect to the optical axis or central axis of the waveguide 125. The deflected laser beam is subjected to divergence and reflected by a mirror surface before incident onto the laser rod 1. This affords a uniform energy absorption distribution in the radial direction. The SSLD of FIG. 8, however, suffers from the problems similar to those of the SSLD of FIG. 6.
There are similar problems in the conventional SSLDs having optical waveguides (waveguide medium or waveguide plates). In addition, the optical lens used for introducing a larger amount of the pumping laser to the waveguide has an optical loss around 5 to 10%, necessitates an accurate position alignment, and thus raises the equipment cost.
The problem that an optimum absorption distribution is difficult to achieve in the SSLD used for mechanical machining will be described hereinafter. In general, for a higher performance of the SSLD in practical applications, it is important to raise the energy conversion ratio, i.e., the ratio of the output power of the SSLD to the power of the pumping laser, and to obtain a higher brightness of the laser beam. The latter may be obtained by a higher focusing capability of the laser beam, i.e., to a small-diameter beam.
For achieving a higher energy conversion ratio, the lasing mode volume in the laser rod should suitably overlap with the absorption distribution of the pumping laser. It is known that the outer periphery of the lasing mode volume in the lasing rod is deviated from the outer periphery of the whole laser rod toward the center of the laser rode even in the case of the maximum thereof. This is due to the diffraction or scattering loss (so-called aperture effect) occurring in the vicinity of the side surface of the laser rod. In other words, the pumping laser absorbed in the vicinity of the side surface of the laser rod is hardly converted to the solid laser energy.
Thus, the pumping laser should be absorbed in the vicinity of the rod center for raising the energy conversion efficiency. In addition, for a higher brightness of the laser beam, a lower-order transverse mode electric field component should be increased, with the laser mode volume being constant. As is well known, since the lower-order transverse mode electric field has a larger electric field lasing component, the pumping laser should be absorbed in more vicinity of the rode center for obtaining a higher gain, which results in dominance by the lower-order mode lasing. Conversely, if the absorbed energy of the pumping laser is reduced in the periphery of the laser rod, a higher-order mode lasing is suppressed, thereby raising the brightness of the laser beam. The conventional SSLDs are evaluated hereinafter from these view points.
FIG. 9 shows absorption distribution of the pumping laser, which is plotted on ordinate against the radial distance from the center of the lasing rod in the SSLD of FIG. 2 plotted on abscissa. As described before, the absorption of the pumping laser should be effected in more vicinity of the center of the laser rod for obtaining a higher energy conversion efficiency and a higher beam intensity. The present inventor, however, found from the experiments the phenomenon that the laser characteristics are in fact degraded, i.e., the laser output power is reduced or the beam is degraded, if the pumping laser beam is excessively concentrated in the vicinity of the rod center at a rate which is four times higher compared to the peripheral area of the laser rod, or if the pumping laser energy is excessively high, e.g., higher than 15xc3x97108 watts/m3.
The above phenomenon results from the fact that the excessively higher ratio of the absorbed pumping laser power between rode center and the side surface of the laser rod results in a higher temperature gradient in the laser rod to raise the thermal lens effect and raise the non linear lens effect of the refractive index. The latter is due to the non-linear temperature dependency of the refractive index. The phenomenon is also considered to result from the fact that the distortion due to the temperature gradient generates a large birefringence, thereby raising the circuit loss.
FIG. 10 shows a desired energy absorption distribution which is designed for the SSLDs of FIGS. 6 and 7. It will be understood from the above reasons that a uniform energy absorption distribution with respect to the radial distance hardly affords a higher energy efficiency or a higher laser intensity. Thus, in FIG. 10, solid line showing the desired absorption distribution is raised in the vicinity of the rod center as compared to the dotted line showing a uniform distribution. In the SSLDs of FIGS. 1 and 2, it is recited that the pumping energy distribution should be raised in the vicinity of the rod center. The commercial dominance by these SSLDs over the other conventional SSLDs is due to the fact that an optimum absorption distribution is not necessarily obtained by the mere higher energy absorption in the vicinity of the rod center.
There is also a problem in the conventional SSLDs that the range of the control in the pumping distribution is narrow. In the conventional SSLD of FIG. 7, it is proposed that the optical axis of the pumping laser be a specified distance apart from the central axis of the solid laser rod in order to prevent an excessive concentration of the pumping laser in the vicinity of the rod center. The pumping distribution recited in this publication is different from the pumping distribution to be proposed by the present invention, however. That is, the deviation of the optical axis in the SSLD of this publication is determined by the focusing device that supports the optical waveguide. Thus, in the SSLD of this publication, if the axial deviation is to be changed by design, the structure of the focusing device must be modified.
The optimum deviation of the optical axis of the pumping laser with respect to the rod center for obtaining an optimum uniformity of the thermal distribution in the solid laser rod depends on the conditions such as the diameter of the laser rod, the density or wavelength of the lasing element, the beam intensity or beam distribution of the pumping laser from the waveguide, and the temperature or flow rate of the cooling medium flowing adjacent to the laser rod surface. Thus, if at least one of these conditions is changed in the conventional SSLD, another focusing device having an optimum axial deviation should be designed. This renders the design and fabrication of the SSLDs to be inefficient and raises the equipment costs of the SSLDs.
In the SSLD of FIG. 8, the optical axis of the pumping laser is deviated from the rod center so that the pumping laser is reflected by the mirror surface disposed around the laser rod for obtaining a uniform absorption distribution. However, this SSLD is not suitable for obtaining a higher efficiency and higher beam intensity by using an optimum absorption distribution, and does not afford an effective laser beam for use in the field of mechanical machining.
In the SSLD of FIG. 1 wherein the pumping laser is directly irradiated, or in the SSLD of FIG. 2 wherein a collimator lens is used for the pumping laser, variations of the incidence angle of the pumping laser cause variations of the pumping energy distribution in the laser rod. This causes an unstable operation of the solid laser or variations of the output characteristics depending on the species of the SSLD.
In addition, the location, beam size or beam shape of the pumping laser may vary depending on the positioning error of the laser diode or the lens thereof. This results from the fact that the optical system for the pumping laser is implemented by an image optical system, which may return the pumping laser beam reflected by the laser rod toward the laser diode. The reflected laser beam may be concentrated on the active layer of the laser diode, thereby rendering the operation of the laser diode unstable or significantly reducing the operational lifetime of the laser diode.
In each of the SSLDs of FIGS. 6 to 8 wherein an optical waveguide is used instead of the image optical system having such a problem, both the thickness of the waveguide and the number of the waveguides should be as small as possible in order to prevent the pumping laser from being leaked from the optical waveguide. This causes a difficulty in introduction of the pumping laser into the waveguide without using the image optical system, which has an inherent optical loss and increases the total optical loss.
In view of the above problems, it is an object of the present invention to provide a diode-pumped SSLD lasing with a higher efficiency.
It is another object of the present invention to provide a diode-pumped SSLD suitable for use in the filed of mechanical machining.
The present invention, in one aspect thereof, provides a solid-state laser device (SSLD) including a cylindrical laser rod for absorbing pumping laser to generate solid-state laser, a cooling member disposed radially outside and co-axially with the laser rod, a mirror member having a substantially cylindrical inner surface disposed radially outside and co-axially with the cooling member, the mirror member having an opening for receiving therethrough pumping laser and a mirror surface for reflecting a portion of the pumping laser passed by the laser rod, the laser rod receiving the pumping laser through the opening and focusing the portion of the pumping laser not absorbed in the laser rod at a focal point substantially on the mirror surface.
In accordance with an aspect of the present invention, for achieving the focal point being substantially on the mirror surface, an optical conjugate relationship is employed between the focal point and the opening with respect to the laser rod by forming a suitable positional relationship.
In the SSLD of the present invention, a small area of the mirror surface disposed at the focal point can reflect the second portion of the pumping laser beam substantially in an opposite direction or a desired direction toward the laser rod. Thus the laser rod can absorb the reflected portion of the pumping laser beam with a higher efficiency, substantially without scattering of the reflected portion of the pumping laser beam toward outside the mirror surface.
More specifically, the configuration that the focal point and the opening are conjugate with each other causes that the pumping laser passing the opening diverges toward the lasing rod, which then focuses the diverged pumping laser by the function of the cylindrical surface thereof to form a substantially parallel ray within the laser rod. The laser rod absorbs a first portion of the pumping laser to emit solid laser, passes a second portion of the pumping laser, and focuses the second portion by the function of the cylindrical surface thereof on the mirror surface of the mirror member, which reflects the second portion substantially in the direction opposite to the incident second portion or desired direction toward the laser rod. The laser rod then receives the substantially entire second portion of the pumping laser without scattering thereof to emit solid laser.
The cooling tube and the cooling medium flowing between the laser rod and the cooling tube may contribute a portion of the lens function of the laser rod.
The term xe2x80x9csubstantially cylindrical inner surfacexe2x80x9d as used herein means that the inner surface has a shape of a circle or a polygon as viewed along the central axis of the laser rod or the inner surface.
It is to be noted in the above configuration that the portion of the pumping laser beam passed by the laser rod during the first incidence can be reflected by the mirror surface substantially at the focal point. Thus, it is sufficient that the mirror surface have a small area. This allows a larger number of openings to be formed in the mirror member without causing leakage of the portion of the pumping laser from the openings.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.